Implantable intracochlear, prosthetic devices have been used in recent years to stimulate auditory nerves so that a person suffering from certain types of deafness may hear. Such prior art devices include U.S. Pat. No. 3,449,768 and Canadian Patent No. 1,115,352.
The human ear can generally be categorized into the outer ear, the middle ear, and the inner ear. A substantial portion of the inner ear is the cochlea, which has a snail-like structure. The cochlea, filled with conductive fluids (perilymph in the scala vestibuli and the scala tympani and endolymph in the scala media) is positioned between the middle ear and auditory nerve endings, and transforms mechanical vibrations (sound) emanating from the middle ear into electrical, auditory nerve signals. Such a transformation is caused by the mechanical vibration of sensory hair cells, more than 15,000 of which are normally present in each cochlea. When these sensory hair cells are damaged, an implantable intracochlear device can be substituted to stimulate the undamaged nerve endings.
Auditory nerve endings are distributed along the entire length of the scala media (endotic space) segment of the cochlea. Those near the basal (larger) end are sensitive to high frequency sound and those near the apical (narrower) end are sensitive to low frequency sound. While exact positioning with respect to frequency is difficult to determine for any given patient or even a group of patients, it is known that beneficial results can be obtained if the electrodes have a predetermined spacing between them, i.e., a relative spacing. To fully duplicate normal hearing, as many of these nerve endings as possible should be stimulated. Accordingly, an implantable device having a plurality of electrodes may be used, each electrode being adapted to transmit over a different frequency band.
The ideal intracochlear device should exhibit a number of, sometimes conflicting, attributes. It should have multiple electrodes to provide a variety of stimulating frequencies. The device should be easy to insert, in that it must have the proper stiffness lengthwise and yet be laterally flexible in order to permit the device to follow the snail-shaped scala tympani section of the cochlea without damaging the sensitive tissue of the basilar membrane therein. This has proved to be a significant problem, and attempts to use solid, rubberized electrodes have not been fully successful because they must be preshaped to conform to the spiral shaped cochlea when they are fully inserted, thus making insertion difficult without trauma to the sensitive basilar membrane on which the organ of Corti rests. If scar tissue develops on the basilar membrane due to trauma, this can interfere with the transmission of electrical signals from the electrode to the auditory nerve cells and remaining hair cells.
Preferably, the device also should be tapered, which is the general configuration of the cochlea.
The device should be biologically inert and be capable of being implanted for long periods of time. The device should be easy to remove and be capable of leaving a tapered channel in which a replacement electrode may be inserted. It should be a "unitary" object where the electrodes are positioned at predetermined optimal spacings, permitting a surgeon to only insert or remove the entire device and not be concerned about the relative positioning of the electrodes during surgery. Further, the device should be capable of minimizing interfering signals.